1. Field of the Invention
The present invention relates to high energy radiation stabilized, irradiated polymeric materials and the method for producing same, and more particularly, concerns a radiation stabilized and sterilized, flexible, non-disclored, semi-crystalline polymer and the method for producing same.
2. Description of the Prior Art
Semi-crystalline polymeric materials, including the polyolefins of which polypropylene is most significant with respect to the present invention, are often employed in producing articles subsequently subjected to irradiation sterilization techniques. For example, in the health and medical field, these sterilizable articles include syringes, tubing and tube assemblies, microbiological plastics, flasks, package film and the like. It is well-known that these semi-crystalline polymeric materials, if not properly stabilized, will discolor and become embrittled after exposure to high energy radiation at levels above 0.1 megarads.
After irradiation has been completed, post-irradiative oxidative degradation has only begun. Free radicals trapped in the crystalline regions of the polymeric materials slowly discharge into the amorphous regions where the bulk of their participation in the branching chain reactions of radiation induced free radical degradation occurs. Therefore, degradation of the mechanical properties of these polymeric materials, such as polypropylene, may not be obvious immediately following irradiation, but as time goes on, brittleness becomes more and more pronounced. Since many medical products made of semi-crystalline polymeric material are subjected to high energy radiation for sterilization purposes, the search goes on to develop satisfactory stabilizers which will render these materials nearly impervious to radiation damage in the region of 0.5 to 6 megarads or somewhat higher, while not imparting unacceptable discoloration to the materials.
Some recent attempts have been made to improve the stability of semi-crystalline polymeric materials so as to reduce embrittlement. For example, U.S. Pat. Nos. 4,110,185 and 4,274,932 disclose flexible, sterilized articles comprised of semi-crystalline polymer which have been irradiated with a sterilizing amount of high energy radiation. Both of these patented inventions, however, rely on the inclusion in the polymer of a mobilizing amount of non-crystalline mobilizing additive. While these materials represent significant improvements, particularly with respect to the embrittlement problem, the inclusion of the mobilizing additive, preferably a hydrocarbon oil, produces some undesirable side effects. In particular, use of oil as the mobilizing additive sometimes causes handling problems, and if the final product is a syringe or the like which usually has graduation marks thereon, the imprinting step is rendered difficult due to the oil in the polymeric material. Thus, while the materials of the aforementioned patents improve or maintain the flexibility of the polymeric material after high energy irradiation, elimination of the undesirable side effects would be welcomed by the manufacturers or users of these materials.
Using stabilizers to protect polyolefins from thermooxidation and photo-oxidation has been known and reported in the literature. For example, Tozzi et al., in "Recent Progress in the Stabilization of Polypropylene Fibers," Textile Research Journal, volume 48, pages 433-436, 1978, describe two new light stabilizers of the hindered amine type for the light stabilization of polypropylene multifilaments. Tozzi et al. suggested that hindered amines apparently do not act directly as radical scavengers, but that the active compounds are the very stable N-oxyl radicals formed by the oxidation of the amine by peroxy radicals or by singlet oxygen. Thus, Tozzi et al. felt that the stabilizing action of these radicals appears to be due to their capacity to trap the less stable radicals formed in the polymer as a result of irradiation and post-irradiative oxidation, with regeneration of N-oxyl radicals and final formation of inactive compounds.
Discoloration and radiolysis of polymeric materials, such as the polyolefins, as a result of high energy radiation is still being investigated not only to completely understand its mechanism, but also to determine a mechanism for its prevention or elimination. Radiolysis as used herein refers to high energy or ionizing radiation which causes the breakage of chemical bonds. It is known that, when high energy radiation dosage, in the order, say, of 3 megarads, is absorbed by polymeric materials, the energy is rapidly distributed through the material by the photoelectric effect, the Compton effect, and even a small amount of pair production. The energetic species formed, e.g., high energy electrons and free radicals, rapidly distribute their energy through the material, causing further ionization and bond breakage with free radical formation. While the energetic species mentioned above individually have very short half-lives and disappear very quickly, the population of free radicals formed in the polymeric material is very long-lived, sustained by the propagation reactions of the transient free radicals. In the amorphous regions of polypropylene, the free radicals rapidly quench. Inside the platelet crystals, however, limited mobility combined with efficient mechanisms of free radical stabilization leads to the storage of some of the energy absorbed by the plastic. As a consequence, free radical degradation of the polymeric material continues for years after irradiation, fed by the slow discharge of free radicals from the capacitor-like crystals. Eliminating or trapping these free radicals which do the bulk of degradation of polymeric materials is one mechanism to produce the desired stabilization.
Moreover, the degradation which accompanies and follows high energy irradiation is much worse in the presence of oxygen. Molecular oxygen is a diradical and, when a radiation-generated free radical is quenched, it is a spin-allowed process for oxygen which happens efficiently, yielding peroxy radicals. The peroxy radicals thus formed can lead to the autocatalytic free radical degradation of the material. Accordingly, free radical trapping is a desired mechanism for the stabilization of polymeric materials such as polypropylene.
If additives in the polymer can react with the radiation induced free radicals to form free radicals which are so stable that they cannot reignite the chain reaction, then these additives should stabilize the polymer to radiation. Even though mechanical properties can be maintained, phenolic stabilizers turn polypropylene very yellow after irradiation due to delocalization of the unpaired spin in the aromatic rings of the products of phenolic free radical chemistry. To eliminate this discoloration, the use of a non-aromatic system such as the hindered amines for forming a free radical trap is desirable.
There is currently much controversy as to the most important mechanisms of protection by hindered amines. Allen et al., "Interaction of a Hindered Piperidine Stabilizer with Hydroxy-Substituted Aromatic Carbonyl Compounds in the Photo-Stabilization of Polypropylene," Journal of Applied Polymer Science, volume 27, pages 2761-2772, 1982, investigated the ultraviolet stability of polypropylene containing Tinuvin 770 hindered amine (Ciba-Geigy Corporation), benzophenone derivatives, and anthraquinone derivatives as well as combinations of these materials. Different frequencies of ultraviolet energy were used and different chemistry was observed to result. Protection against near ultraviolet light (low energy) was better for Tinuvin 770 with UV 531 (2-hydroxy-4-n-octoxybenzophenone) or 1-hydroxy-anthraquinone than for Tinuvin 770 alone. Protection against the more energetic ultraviolet light was better for Tinuvin 770 with benzophenone than for Tinuvin 770 alone. Interestingly, the work of Allen et al. disclosed reversals in additive package efficacy depending upon the energy of the ultraviolet photons used. Their conclusion, that peroxide decomposition is the most important mechanism of stabilization by Tinuvin 770 to ultraviolet of wavelengths greater than 340 nm was due to the observation that Tinuvin 770 prevents polymer degradation by near UV but not by far UV (less than 340 nm). The work of Allen et al. showed that benzophenone improved stability of a T-770 mix to far UV (&lt;340 nm) but not near UV, for which the effect was antagonistic. When 1-hydroxyanthraquinone was similarly employed with T-770 mixes, the effect of frequency was reversed. They clearly demonstrate by their work that the mechanism of polymer degradation, and thus the efficacy of candidate stabilizer packages, is dependent on the energy of the photons of irradiation. Therefore, packages which give high stability to solar radiation will not necessarily work for UV less than 340 nm or for ionizing radiation.
Other photostabilizers related to polyalkylpiperidine derivatives have been disclosed in U.S. Pat. Nos. 4,314,933 and 4,344,876. The subject matter of these two patents is related and deals with stabilizing lacquer resin against light, moisture and oxygen using any of a large variety of hindered amines. One of the listed hindered amine compounds has a 2-hydroxybenzophenone moiety. However, in U.S. Pat. No. 4,314,933, at col. 18, starting on line 30 thereof, it was pointed out that more stability to UV light can be achieved by adding UV absorbers and other conventional stabilizers. This is not a statement of synergism but merely a reiteration of a very old and understood principle of light stabilizers; the more UV excluded from the plastic, the less UV-induced degradation will occur. Not all absorbers, however, will cooperate with the hindered amine. For example, many of the organic nickel compounds suggested for use with hindered amines are hydroperoxide decomposers and, just like thioester, their addition to a hindered amine polypropylene will decrease its stability. It has been demonstrated that synergism can be achieved with some of the materials listed in U.S. Pat. No. 4,314,933, while antagonism occurs with slightly different chemicals of the same type. Thus, the patentee has made broad, general characterizations, but has failed to state a synergistic effect between the hindered amine and any specific compounds.
Carlsson et al., in the Journal of Applied Polymer Science, 16 615 (1972) includes resorcinol monobenzoate (RMB) in a list of stabilizing additives for polypropylene films. U.S. Pat. No. 3,546,161 discloses stabilization of polyolefins toward light with RMB and an alkyl amine.
Despite the aforementioned investigations and patented inventions, the unsolved problems of the radiation degradation, oxidation and stabilization of polymers still exist. It is toward the solution of these problems, or at least to an improvement thereover, that the inventive efforts of the present invention have been directed.